1. Field of the Invention
The present invention relates, generally, to a method for treating and regenerating a spent polymeric ion exchange resin. In particular, the present invention relates to a method for removing unwanted impurities from ion exchange resins in a safe, and environmentally responsible manner and more particularly to a method for regenerating the ion exchange resin which has been saturated with large, poorly hydrated anions, particularly perchlorate (ClO4xe2x88x92) anions. The disclosed method is, however, generally applicable to many other similar species such as rhenates (ReO4xe2x88x92), arsenates (AsO2xe2x88x92), tungstenates (WO4xe2x88x92), pertechnetates (TcO4xe2x88x92) and the like.
2. State of the Art
Perchlorate-based propellants are widely used in both civilian and government operations, typically mining, demolition, and aerospace applications subsequently large volumes of perchlorate-containing compounds have been disposed of in the environment since the 1950s. Past methods for grossly disposing of these materials have consisted largely of open-air burning, detonation or simply aqueous dissolution. The perchlorate (ClO4xe2x88x92) anion, therefore, has origins as a contaminant introduced into the environment from the uncontrolled disposal of the solid salts of ammonium, potassium, or sodium perchlorate. Unfortunately, these salts are very soluble in water and have been found to be chemically inert in many natural redox reactions and non-complexing in their interactions with typical metal ions found in the environment. These properties make the perchlorate ion exceedingly mobile in subsurface soil environments and allow it to persist for many decades under typical groundwater and surface water conditions. The presence of this material in the environment poses a potential threat to ecological receptors and the public.
The California Department of Health Services has proposed setting an action level of 18 xcexcg/L (ppb) for perchlorate in groundwater, a level only recently analytically detectable. Drinking water wells owned by the city of Redlands, in Riverside County California are facing increasing intrusion of perchlorate from a known plume of perchlorate contamination in the local aquifer. Treatment technologies capable of specifically removing very low levels of ClO4xe2x88x92 (xcx9c50 ppb) from groundwater are, therefore, urgently needed.
Water utilities, in particular, need treatment methods that can reliably reduce ClO4xe2x88x92 concentrations to low or non-detectable levels. ClO4xe2x88x92 ions are nonvolatile and highly soluble in water, and cannot be removed by conventional filtration, sedimentation, or air-stripping methodologies. To be useful, a drinking-water treatment method must be cost effective, acceptable to regulatory agencies and the public, and generate a minimum amount of secondary waste. In addition, a treatment method must not be adversely affected by other naturally occurring components in the water and it must not contribute to other water quality or distribution system problems. Urbansky, E. D., xe2x80x9cPerchlorate chemistry: Implications for analysis and remediation,xe2x80x9d Bioremed. J. (2), pp. 81-95 (1998) has outlined some candidate treatment technologies and pointed out the drawbacks of each. Membrane-based technologies, such as reverse osmosis, were thought impractical as were conventional, non-specific anion exchange resins. The cationic chemical nitron precipitates perchlorate efficiently and might be suitable for remediation of high concentrations of perchlorate. However, the cost and waste disposal issues render this technology unacceptable. Given the chemical inertness of ClO4xe2x88x92 to reduction reactions and the small concentrations involved, it seems clear that chemical reduction is not feasible. Bioremediation methods appear to be the most economical and technically feasible remediation methods at the moment. However, a method based on live bacteria and added nutrient gradients is unlikely to be embraced by government, industry, and water utilities. Furthermore, current biological processes are time intensive, batch methods while the current need is for in-line wellhead amenable technology.
Highly selective anion exchange resins offer considerable advantages over conventional (non-selective) resins in, the treatment of perchlorate contaminated groundwater. The Oak Ridge National Laboratory (ORNL) has developed anion-exchange resins that have a bias for the sorption of large poorly hydrated anions such as pertechnetate (TcO4xe2x88x92) and perchlorate (ClO4xe2x88x92) anions from contaminated groundwater (Brown et al., 1995; Brown et al., 1997; Brown et al., 1996). Highly selective bifunctional anion-exchange resins were developed in U.S. Department of Energy (DOE) sponsored work for the sorption of TcO4xe2x88x92 from contaminated groundwater at the Paducah Gaseous Diffusion Plant site, Kentucky (Gu, et al., 1998). Groundwater at several DOE sites is contaminated with the radionuclide Tc-99 which exists in the chemical form of TcO4xe2x88x92 anions in oxygenated environments (Gu et al., 1996; Gu and Dowlen, 1996). Pertechnetate, like ClO4xe2x88x92, is highly mobile in underground aquifers. The concentration of TcO4xe2x88x92 ion is 4 to 6 orders of magnitude lower than the other anions found in groundwater, such as chloride, sulfate, nitrate, and bicarbonate.
While these highly selective anion exchange resins provide useful service, their manufacture and disposal costs make it imperative that a method be found to regenerate the resin in order that its useful service-life be extended. Regeneration would consist of xe2x80x9cwashingxe2x80x9d the resin in order to remove or xe2x80x9cre-exchangexe2x80x9d the perchlorate anion with a similar chloride anion source as was present in the virgin resin. Unfortunately, the very properties which make these resins so attractive also make them difficult to recondition in a manner which does not itself generate an environmentally unacceptable waste stream. In particular, because the exchange sites of the spent resin has substantial hydrophobic character, the use of an aqueous regeneration method would be thought to be less effective or impossible, suggesting use of an organic solvent instead. However, since most such solvent materials are themselves a waste hazard which would require remediation and disposal such an approach serves simply to substitute one waste stream for another.
What is needed therefore, is a method for regenerating these highly selective resins in such a way as to return them to an xe2x80x9cas-newxe2x80x9d condition without exacerbating the remediation problem by introducing an organic solvent waste stream.
This invention relates generally to a method for regenerating an anion exchange absorbent contaminated with perchlorate (ClO4xe2x88x92) thereby restoring said resin to a near original condition. More specifically, the present invention relates to a novel method for regenerating a bifunctional anion exchange resin by providing an efficient way for removing the perchlorate anion form the spent resin.
It is therefore an object of this invention to provide a method for removing a tightly bound ion from hydrophobic sites on an organic ion exchange resin medium without the use of a (typically petroleum-based) non-aqueous organic backwash.
Yet another object of the invention is the use of liquid, or supercritical, carbon dioxide as an exchange medium to solvate the perchlorate anion in favor of the quaternary chloride anion and thereby bringing an organic ammonium chloride into close proximity with the perchlorate anion bound at the resin exchange site in order to allow a second exchange reaction to occur.
It is a further object of this invention to provide a method for regenerating an ion exchange resin which includes a non-aqueous backwash wherein the liquid carrier medium is other than an fluid organic hydrocarbon.
It is another object of this invention to provide a method for regenerating an ion exchange resin which includes a supercritical fluid backwash.
Another object of this invention is to provide a solvent composition for removing anions selectively exchanged on said ion exchange resin.
Yet another object of this invention is to provide a solvent composition which includes liquid, or supercritical, carbon dioxide as a carrier medium.
Another object of this invention is to provide a solvent composition which includes supercritical carbon dioxide as the carrier medium.
Yet another object of this invention is to provide a solvent composition which includes a surfactant.
Another object of this invention is to provide a solvent composition which includes one or more surfactants taken from those used in the water treatment industry.